1. Field of the Invention
The present invention relates to an improved multiple-pass product treating device useful for drying a variety of particulates such as wood furnish and agricultural products. More particularly, it is concerned with such a device which achieves substantially increased efficiency through the use of an internal flow path arrangement serving to direct incoming initially wet product through a premixing zone at a relatively slow speed and thence along a serpentine flow path beginning in an innermost relatively small cross-sectional area and proceeding to passageways of successively larger cross-sectional areas until dried product is removed from the apparatus.
In other aspects of the invention, apparatus is provided for the introduction of relatively dry ambient air into the outermost dryer drum to reduce the partial pressure of water vapor of the air currents passing through the dryer to increase final stage drying in the apparatus.
In this fashion the velocity of air currents within the device is first increased and then decreased as the currents pass therein, whereby the velocity differential between such air currents and the saltation velocities of the particles being dried is maintained for maximum drying effect.
2. Description of the Prior Art
The drying of wood or agricultural particulates in a multi-stage dryer is dependent upon a large number of factors, e.g. the type of product to be dried, the initial moisture content thereof, particle geometry, variable ambient conditions, dryer configuration and fuels being employed.
In general, however, the drying process involves several distinct phases or stages. That is to say, most hygroscopic materials exhibit several distinct drying rate periods as they pass through a multi-pass dryer. Initial drying is accompanied by a warming of the material and its attendant moisture. The drying rate increases during this initial period, while the moisture content drops to a value which signals the beginning of a constant rate period of drying. During the constant rate period moisture is evaporated from the surface of product particles at a steady rate until the surfaces are no longer entirely wet. Thereafter, a falling-off period obtains where the drying rate decreases because of the increasing difficulty of moving internal product moisture to the particle surfaces where it can be taken up and moved away. Finally, the product moisture is reduced to a point where an equilibrium is established with the surrounding atmosphere.
Conventional three-pass dryers include an elongated horizontal, axially rotatable body having an outer drum and a series of concentric smaller diameter drums within the outer drum. The drums are in communication with each other and define a serpentine flow path within the dryer. Such dryers are provided with a product inlet oriented for directing initially wet product and hot drying air into the innermost, smallest diameter drum, whereupon the product is conveyed via induced draft current through the outer drum until it reaches a passageway defined by the outer drum and the next inboard drum. At this point the product is in its final fried condition and is delivered for further handling or collection. Thus, conventional three pass cylindrical dryers utilize comparatively high air velocities and temperature conditions in the innermost drum (first pass) where the incoming products are the heaviest and the wettest. Lower air velocities and lower temperatures obtain in the intermediate drum (second pass), and even lower velocities and temperatures exist in the outer drum (third pass). In practice, however, the relatively high air current velocity conditions in the first pass of a conventional dryer cause the wet product particles to be quickly driven away from the heat source, and there is consequently a reduced opportunity for adequate heat transfer and evaporation. In subsequent passes with lower air current velocities, the particles may settle out because the prevailing air current velocities fall below the saltation velocity of the product (i.e. the minimum air current velocity needed to pick up and convey product at a given moisture level). Thus, plugging of the dryer may occur, particularly at high product flow rates, and at best the product only moves at a rate determined by the forward velocity of the slowest moving (largest) particles. The result is that the flow rate is decreased and this inevitably has an adverse effect on drying efficiency.